Skills such as whirlwind and those learned from spell books (frostbite, healing rain, disorient, dash, shell, etc.) disappear after use *eventually*. You have to obtain a new spell book and read it in order to get whirlwind back, or you can choose a different spell book (as a low level I advise Dash, as it allows you to escape quickly, but try some out). You can also use your myst to open up new spell book slots, so you’re not limited to just one spell.
There is a random chance that you’ll lose the spell upon use, and a random chance that you’ll break the spell book when you read it – possibly before you learn the spell. As you gain experience in the research skill the odds get better.
Spellbooks can be obtained from other players or from the Skeleton King (do not attempt this alone) or from chests found in the underworld.